As a conventional cap having a two-piece structure, for example, a cap disclosed in Patent Document 1 shown in FIG. 7 is known. Specifically, the cap includes a cap main body 4 and a gasket 5. The cap main body 4 has a top wall 6 and a skirt wall 7 pending from the outer circumference of the top wall 6, and the cap main body 4 in which the skirt wall 7 is formed with an internal thread 9 that comes in screw engagement with an external thread 10 of a container mouth part 3, the gasket 5 made of synthetic resin for hermetically sealing the container mouth part 3, and a tamper evidence band 13 connected below the skirt wall 7 via breakable bridges 12 are provided.
The gasket 5 includes a top plate 17 that comes into abutment with a distal end part 3a of the container mouth part 3 to cover the distal end part 3a, and an annular inner leg 18 pending from the top plate 17 and coming into close contact with an inner circumferential face 3b of the container mouth part 3. Further, on the outer circumferential edge of the top plate 17, an annular bent part 19 bending downward with respect to the top plate and having an outer circumferential face inclined to extend outwardly is continuously provided.
In the above cap, even when a container is stored at a low temperature (for example, 4 to 5° C.), it is possible to prevent leakage of airtightness inside the container caused by a doming phenomenon. This is because when a doming phenomenon occurs, an upper end part of the circumferential wall of the cap main body 4 contracts toward the center, and a pushing force toward the center direction of the container mouth part 3 is exerted on outer circumferential edge of the gasket 5, so that the bent part of the gasket 5 is pushed against an outer circumferential face 3c of the container mouth part 3 by this pushing force.    Patent Document 1: Japanese Patent Publication No. 3936487